1. Field of Invention
The invention relates to a machine-detectable security marking with an increased level of proof against forgery, to the production of the security marking and to a security system comprising this security marking.
2. Description of the Related Art
The growing technical maturity of color copiers is leading to copies whose color, resolution and quality are increasingly difficult to distinguish from the originals. As protection against forgery using color copiers or scanners, the use of optically variable elements is becoming more and more widespread as a security marking for data carriers. A common feature of such markings is that the color or brightness they present varies depending on the conditions under which they are illuminated and observed. The most common optically variable security markings include diffraction gratings, holograms, interference coatings, metameric inks and polarizing coatings.
DE 195 41 028 describes effect coating materials for articles that are to be coated, comprising liquid-crystal pigments, which carry a label for recognition and characterization of the article. Labeling in this case is via the surface structure of the pigments with a layer thickness of  greater than 0.5 xcexcm, in accordance with the barcode principle, or by defined spectral characteristics or by color patterns. Also described are processes for producing the effect coating material and the marked pigments.
DE 3942663 discloses data carriers with an optically variable security element. The security element comprises a liquid-crystal material comprising liquid-crystal polymer in oriented form at room temperature, which is present as a solid. DE 3942663 discloses the use of liquid-crystal polymer systems having lattice constants of 300-1000 nm, which with an average refractive index of normally 1.5 for liquid-crystalline materials gives a reflection wavelength of from 450 to 1500 nm for the liquid-crystalline material. Also disclosed are the optional combination of the LC system with xe2x80x9cconventional inksxe2x80x9d, semifinished articles and processes for producing the security elements, and methods and mechanical test setups for detecting color, color flop and polarization of the security elements. The center wavelength test performed therein is inadequate for an increased level of proof against forgery. This is demonstrated in the comparative example of the present application.
DE 19544130 discloses optically variable security elements comprising at least two printed-on layers. The first printed-on layer is structured, with a color contrasting with the data carrier, and the second printed-on layer is configured with optically variable pigments, with little or no mass tone, and is at least partly superimposed on the first printed-on layer. A description is also given of preparation techniques for optically variable security elements of this kind.
It is an object of the invention to provide security markings whose level of proof against forgery is greater than that of known security markings.
This object is achieved by a security marking comprising liquid-crystalline material with chiral phase, wherein present within the security marking are at least two liquid-crystalline materials with chiral phase which differ in at least one property selected from the group consisting of handedness, color, and color flop, and which are arranged in the form of a structured or unstructured marking, a multilayer marking, in the form of liquid-crystalline pigments in a liquid-crystalline matrix, wherein there is at least one liquid-crystalline material with chiral phase whose reflection band has been broadened by means of special techniques, or wherein there is at least one liquid-crystalline material with chiral phase which has a defined three-dimensional arrangement.
The security marking of the invention can only be reproduced with a level of complexity which is substantially higher than that of prior art markings, which leads to a security marking having an increased level of proof against forgery.
The invention also relates to articles labeled with the security marking of the invention, to processes for preparing the security markings, and to a security system employing these markings.